Mechanical and viscoelastic properties of soybean oil ...

Mechanical and Viscoelastic Properties of Soybean OilThermoset Reinforced with Jute Fabricsand Carded Lyocell Fiber

Kayode Adekunle,1 Christian Patzelt,2 Adib Kalantar,1 Mikael Skrifvars1

1School of Engineering, University of Boras, SE-501 90 Boras, Sweden2Automotive Engineering, University of Applied Sciences, Westsachsische Hochschule Zwickau, 08012 Zwickau,Germany

Received 16 August 2010; accepted 18 February 2011DOI 10.1002/app.34360Published online 29 June 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Composites and hybrid composites weremanufactured from renewable materials based on jutefibers, regenerated cellulose fibers (Lyocell), and thermo-setting polymer from soybean oil. Three different types ofjute fabrics with biaxial weave architecture but differentsurface weights, and carded Lyocell fiber were used asreinforcements. Hybrid composites were also manufac-tured by combining the jute reinforcements with the Lyo-cell. The Lyocell composite was found to have bettermechanical properties than other composites. It has ten-sile strength and modulus of about 144 MPa and 18 GPa,respectively. The jute composites also have relativelygood mechanical properties, as their tensile strengths andmoduli were found to be between 65 and 84 MPa, and

between 14 and 19 GPa, respectively. The Lyocell-rein-forced composite showed the highest flexural strengthand modulus, of about 217 MPa and 13 GPa, respectively.In all cases, the hybrid composites in this study showedimproved mechanical properties but lower storage modu-lus. The Lyocell fiber gave the highest impact strengthof about 35 kJ/m2, which could be a result of its mor-phology. Dynamic mechanical analysis showed that theLyocell reinforced composite has the best viscoelasticproperties. VC 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122:2855–2863, 2011

Key words: mechanical properties; renewable resources;impact resistance; biofibers; thermosets

INTRODUCTION

Renewable materials are being sought after due to thefact that they are sustainable and environmentalfriendly. Government policy on reducing the emissionof greenhouse gases is the main drive toward sustain-ability. Many researchers are working on biobasedmaterials to improve the mechanical properties and topossibly discover a wider range of applications.

There have been many reports on the reinforce-ment of biodegradable thermoplastics with natural/plant-based fibers.1–5 Preparation and characteriza-tion of biocomposite materials from natural fibersand natural matrices has been reported by Takaha-shi et al.,6 Alix et al.,7 and Tran et al.8 A comprehen-sive review of biofibers and biocomposites has alsobeen published by John and Thomas.9 Carrilloet al.10 reported the properties of a conventionalthermoplastic reinforced with Lyocell fiber; also,hybrid composites of jute and man-made cellulosefiber with polypropylene have been reported by

Khan et al.,11 but quite little has been reported onhybrid woven fabric/Lyocell fiber-reinforced bio-based thermosetting polymers.Textile-reinforced composites based on natural

fibers have been studied by many research groupsin recent years due to their good mechanical per-formance, excellent drape ability, easy handling,excellent integrity, conformability for advanced struc-tural applications, and reduced manufacturing cost.12

Woven fabrics have been found to be better than non-woven fibers as reinforcements, because the weavearchitectures of woven fabrics affect the permeability,and the mechanical and fracture properties of thecomposite.12 On the other hand, nonwoven mats withaligned fibers are of interest as they have no crimpand are of low cost. Textile structural composites arefinding use in various high-performance applica-tions.13 Bledzki and Zhang14 have reported the use ofjute fabrics as reinforcement in the preparation ofcomposites. Different cellulose fibers have been stud-ied by many researchers as reinforcement in variousmatrices.15,16 The mechanical properties of jute-wovenfabric-reinforced polyester composites have been stud-ied by Munikenche et al.17 and Ahmed and Vijaya-rangan18,19 and Wambua et al.20 also discussed theproperties of flax, hemp, and jute fabric-reinforced

Correspondence to: K. Adekunle ([email protected]).

Journal of Applied Polymer Science, Vol. 122, 2855–2863 (2011)VC 2011 Wiley Periodicals, Inc.

polypropylene. Various treatments can be done tothese natural fibers to improve their wettability andconsequently improve the fiber–matrix adhesion inthe resulting composite. Many authors have doneextensive work on natural fiber treatment.21–23

Lyocell is a regenerated cellulose fiber derivedfrom bleached wood pulp. Lyocell is obtained by asolvent spinning technique, using N-methylmorpho-line N-oxide as the solvent. The spinning process issimpler and more environmentally sound than the Vis-cose spinning process, as it uses a solvent that is lesstoxic than the carbon disulfide used in the Viscoseprocess, it can also be recycled in the manufacturingprocess. The regenerated cellulose fibers are of interestin structural composites, as they represent chemicallypure cellulose fibers with an even quality and per-formance that cannot be achieved with mechanicallytreated natural fibers such as flax and hemp.

A hybrid biobased composite is a combination ofthe individual characteristics of at least two differenttypes of natural fiber reinforcements in a singlerenewable matrix. The properties of hybrid compo-sites are a weighed sum of the individual compo-nents, but there may be a more favorable balancebetween the inherent advantages and disadvan-tages.24 This means that the attributes of one type offiber can complement ones lacking in the other.24 Asa result, a balance in cost and performance can beachieved through proper material design.

In this study, woven jute fabrics and carded Lyo-cell fiber mat were used as reinforcements in themethacrylic anhydride-modified soybean oil (MMSO)

thermoset, and the properties of the composites wereanalyzed by tensile and flexural testing, testing ofimpact resistance, and dynamic mechanical thermalanalysis (DMTA). Microstructural analysis was donewith scanning electron microscopy (SEM).

EXPERIMENTAL

Materials

MMSO was used as matrix in the preparation of com-posite. The synthetic pathway for chemical modifica-tion of the MMSO is shown in Figure 1. The matrixresin was synthesized according to a method pub-lished earlier.25 Three different types of jute andcarded Lyocell reinforcements were used in the prepa-ration of composite: Lyocell fiber (Tencel Lenzing Lyo-cell, 1.7 dtex, 30 mm cut length) was supplied byLenzing AG, Business Development and InnovationTextiles (Lenzing, Austria). The Lyocell fiber wascarded and needled to obtain a nonwoven mat. Biax-ial-woven jute fabrics with surface weights of 240, 300,and 100 g/m2 were all supplied by HP JohannessonTrading AB (Svalov, Sweden; Fig. 2 and Table I). Thefree radical initiator, tert-butylperoxybenzoate was sup-plied by Aldrich Chemical Company (Wyoming, IL).

Carding and needling of the Lyocell fiber

The carding of the cellulose fiber was done with acylindrical cross-lap machine supplied by Cormatex(Prato, Italy). The cellulose fibers were separated

Figure 1 Chemical modification of methacrylated soybean oil (MSO) to give methacrylic anhydride-modified soybean oil(MMSO).

2856 ADEKUNLE ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

manually and fed into the carding machine. The fre-quency of the trolley was 40 Hz, which was equiva-lent to 7.5 m/min at the outlet. The needling wasdone at a frequency of 200 cp/min and the feedingrate was 1.5 m/min, while the depth of the needlewas 8 mm. The needling machine was supplied byCertec (Sourcieux-les-Mines, France)

Needle penetration depth and frequency contrib-uted to entanglement of the fiber. The frequency isrelated to the feeding speed: the more the needling,the stiffer the material obtained. In this case, the nee-dling was done three times. The surface weight ofthe carded Lyocell mat was 525 g/m2.

Composite preparations

The jute fibers were washed with 4% sodium hy-droxide solution for 1 h and dried overnight; theywere post-treated by heating at 105�C for 1 h. As thenatural fibers bear hydroxyl groups from celluloseand lignin, they are amenable to modification. Thehydroxyl groups may be involved in the hydrogenbonding in the cellulose molecules, thereby reducingthe activity toward the matrix.26 Chemical modifica-tions may activate these groups or introduce newmoieties that can effectively interlock with the ma-trix.26 The Lyocell fiber was not washed. The matrixused was MMSO blended with 2 wt % tert-butylper-oxybenzoate as free radical initiator.

Composites and hybrid composites were preparedfor the purpose of comparison. Composite laminateswere made by stacking sheets of fiber mats as a pre-form, and resin impregnation was done by handspraying. The prepreg was then inserted in a moldand compression molded at 160�C for 5 min using apressure of 40 bar to get an approximate thicknessof 3.5–3.7 mm for the composites and a thickness of3.9–4.3 for the hybrid composites. The hybrid compo-sites were made by sandwiching plies of Lyocell fiberin between the jute fibers. The jute/Lyocell ratio wasmaintained at approximately 60 : 40 by weight (seeTable II) and compression molded as explained ear-lier. The direction of the carding was taken to be thedirection of the fiber in the case of the Lyocell fiber,whereas there was no specific fiber direction in thecase of the jute woven fabrics because all the samplesare biaxial. The hot press was supplied by RondolTechnology (Staffordshire, UK). The fiber/resin ratiowas about 60 : 40 wt% in all cases.

Figure 2 Jute woven fabrics. [Color figure can be viewed in the online issue, which is available atwileyonlinelibrary.com.]

TABLE ICharacterization of Jute Fabric Reinforcements

Fibers

Yarn per10 cm(weft)

Yarn per10 cm(warp)

Twist (turnsper inch)

Surfaceweight(g/m2)

W1 32 40 4–5 240W2 46 50 4–5 300W3 15 17.5 4–5 100

MECHANICAL AND VISCOELASTIC PROPERTIES OF SOYBEAN OIL 2857


Composites reinforced with woven fabric of240 g/m2 surface weight were denoted W1 (seeTable I), those with woven fabric of 300 g/m2 surfaceweight were denoted W2, those with woven fabric of100 g/m2 surface weight were denoted W3, and theLyocell-reinforced composite was denoted L. Thehybrid composites from the woven fabric/Lyocellwere denoted as follows: W1L, W2L, and W3L.

Characterization

Mechanical characterizations of the composites weredone by tensile, impact, and flexural testing. Toobtain high-quality test specimens, cutting of allspecimens was done with a laser cutting machine.This machine was a Laserpro Spirit (50-W sealedCO2, DC servo control, and 860 � 460 mm2 workarea). The Lyocell composite specimens were all cutin the carding direction.

The tensile testing was performed according to theISO 527 standard test method for fiber-reinforcedplastic composites, with a universal testing machine(H10KT; maximum capacity 10 kN; Tinius Olsen,Salford, UK). For each composite laminate, 10 speci-mens were analyzed.

The flexural testing was performed according toISO 14125, using the same testing machine. At leastseven specimens of every material were tested.

Impact testing was done on the composite laminatesto determine the Charpy impact strength of the un-notched specimens. This was evaluated in accordancewith ISO 179 using a Zwick test instrument. Ten speci-mens in total were tested to determine the meanimpact resistance. The samples were tested edgewise.

The time–temperature dependency of the mechani-cal properties was determined by DMTA, using aQ-series TA instrument dual cantilever supplied byWaters LLC (Newcastle, DE). The temperature rangewas from 30�C to 150�C, and the frequency was 1 Hz.

SEM analysis was performed on the tensile-fractured specimens. Gold coating of the fracturedspecimens was done with a sputter coater (S150B) in

argon gas and at 3 mbar. The specimens were lateranalyzed with DSM 940A equipment, supplied byBlue Lion Biotech LLC (Snoqualmie, WA). Theequipment was run at 2.0 kV, 8.0 � 500 mm2.

RESULTS AND DISCUSSION

Tensile properties

The Lyocell fiber-reinforced composite L was usedfor comparison. The tensile strength of this compos-ite was found to be almost double when comparedwith those of the other composites (see Fig. 3).Although the Lyocell is nonwoven, the three timesneedling of the carded mat may have impartedhigher mechanical properties to the fiber which thengave better tensile strength to the resulting compos-ite. All three jute fabrics had plain-weave architec-ture but the major difference was the distancebetween the adjacent roving wefts and warps [Fig.2(a–c)]. The difference could be seen in the tensilestrengths; however, because they had differentstrengths—which may have been a result of the weftand warp distances. The fiber weight percentagewas the same for all composites made. Althoughwoven fabrics gain integrity from interlacing ofwarp and weft, interlacing induces waviness oftows, which in turn imparts crimp—and this mayaffect the mechanical properties of the composite. Inter-lace points have been identified as one of the weakestpoints in most woven fabric composite systems, andinterlace points are higher in the case of plain-weavearchitecture. This could lead to the presence of voidsand fiber distortion at the interlace gap.24 The Lyocellcomposite (L) had a superior tensile strength of about144 MPa, which indicated that this composite was thetoughest and strongest (see Fig. 3). The direction ofcarding was taken to be the fiber direction in the caseof the Lyocell fiber, and this might also have contrib-uted to the better tensile properties.However, there was a huge effect of Lyocell

hybridization on all the jute composites; for instance,

TABLE IIComposite Laminate Compositions

Composites

Numberof juteplies

Numberof Lyocell

plies

Totalnumberof plies

Weightof

preforms (g)

W1 14 0 14 82W2 10 0 10 80W3 27 0 27 81L 0 6 6 81W1L (hybrid) 8 3 11 82W2L (hybrid) 5 3 8 80W3L (hybrid) 18 3 21 82

Percentage weight (wt %) jute/Lyocell was 60/40 andthe fiber–matrix ratio was approximately 60 : 40.

Figure 3 Tensile strength of the jute and Lyocell compo-sites compared with the jute/Lyocell hybrid composites.



the tensile strength of the hybrid composites (W1L,W2L, and W3L) was increased by 19%, 48%, and26%, respectively (Fig. 3). The improvement in ten-sile properties was conspicuous in the hybridizationwith the woven fabrics. This may be due to the mor-phology of the Lyocell fiber.

The percentage elongation (Fig. 4) for the jutefiber-reinforced composites was about 0.5% on theaverage. The percentage elongation for the Lyocell-reinforced composite (L) was about 1.3%; this was tobe expected, due to the morphology of the regener-ated cellulose fiber. This is evident in the higher ten-sile and flexural strengths and also in the highimpact resistance. The hybrid composites showedimprovement in elongation properties due to theeffect of the Lyocell fiber. However, Lyocell fiberimparts toughness to the composites.

In this work, equal fiber weight percentage was usedin all cases to achieve reproducibility (see Table II). Thetensile modulus of the composites was relatively high(between 14 and 19 GPa; Fig. 5) and when the standarddeviation is taken into consideration, one can say that allthe composites had almost equal stiffness.

However, hybridization of the woven fabric W2(300 g/m2) with Lyocell fiber showed an appreciable

increase in tensile modulus from 14 to 17 GPa (seecomposite W2L in Fig. 5). Hybridization of Lyocellfiber with woven jute fabrics could impart toughnessto the manufactured composites.

Flexural properties

Figure 6 shows the flexural strength of the compo-sites. The Lyocell-reinforced composite L showedthe highest flexural strength of about 217 MPa. Thehybrid Lyocell/jute fiber composite W2L showedincreased flexural strength but the effect of hybrid-ization was negligible in composites W1L and W3L(Fig. 6). Misalignment of fibers usually occurs inwoven fabrics, especially when there is appreciabledistance between the adjacent weft and warp, andthis could be the possible reason for the drop in flex-ural strength in composite W3L.The Lyocell-reinforced composite had the highest

flexural modulus of about 13 GPa (Fig. 7), buthybridization of Lyocell fiber with other jute fibershad a negligible effect on the flexural modulus ofthe resulting hybrid composites. When the individ-ual hybrid composites (W1L, W2L, and W3L) werecompared, however, the thickness of the externalplies (see Table II) may have contributed to the

Figure 6 Flexural strength comparison of the compositesand the hybrid composites.

Figure 4 Percentage elongation of the composites and thehybrid composites.

Figure 5 Tensile moduli of the jute and Lyocell compo-sites compared with those of the jute/Lyocell hybridcomposites.

Figure 7 Flexural modulus comparison of the compositesand the hybrid composites.



flexural modulus of each composite. Hybrid compo-sites W3L, W1L, and W2L had thicker outer plies inthat order, and thus flexural modulus of 8, 7, and6 GPa, respectively.

Impact resistance

Figure 8 represents the Charpy impact resistance(energy absorbed/cross-sectional area). The jutecomposites (W1, W2, and W3) showed relatively lowimpact resistance between 11 and 13 kJ/m2, whichcould be attributed to good fiber–matrix adhesion.Higher fiber–matrix adhesion resulted in shorteraverage pull-out lengths, and therefore caused lowerimpact resistance or strength. The results from theflexural tests showed higher flexural strengths forthe jute composites of between 120 and 137 MPaand flexural moduli of between 5 and 8.5 GPa.

The matrix used in the preparation of composite(MMSO) has a higher cross-linking density due to ahigher number of reactive double bonds in themolecular structure. The neat MMSO resin is alsovery brittle.

The Lyocell composite (L) had the highest impactresistance (36 kJ/m2; Fig. 8), which indicated a lon-ger fiber pull-out length, and this could be due tothe structural and morphological nature of the Lyo-cell fiber (regenerated cellulose fiber). The hybridiza-tion of the jute fibers with Lyocell fiber increasedthe impact resistance of the composites slightly tobetween 14 and 15 kJ/m2 (see composites W1L,W2L, and W3L in Fig. 8).

Dynamic mechanical thermal analysis

Storage modulus is a measure of the elastic responseof a material, and in this study, the Lyocell compos-ite L showed the highest storage modulus (Fig. 9).This indicates that it has better elastic properties thanthe other composites. Loss modulus is a measure ofthe viscous response of a material. The Lyocell com-posite L had the highest loss modulus (Fig. 10). The

results indicate that Lyocell-reinforced composite Lhad the best viscoelastic properties of all the manufac-tured composites and hybrid composites. Viscoelas-ticity is the ability of a material to exhibit both elasticand viscous behavior. The better properties of theLyocell composite L could be attributed to its reinforc-ing effects, which was also supported by the othermechanical analyses.Hybridization with Lyocell fiber reduced the stor-

age modulus of all the jute composites (Fig. 9), andthis could be due to delamination during constantheating and deformation for about 1 h in the equip-ment and the possibility of mismatch in the hybridcomposite structure. Lyocell fiber and jute fabricswere combined in this case, and a microstructuralanalysis of a transverse section of the specimenmight give a better explanation.The glass transition temperature can be deter-

mined by the tan d curves; see Figure 11. The tan dpeak for the Lyocell composite corresponds to thetemperature at 146�C, while the other jute compo-sites (W1, W2, and W3) have their Tg at about130�C. The effect of hybridization reduced the glasstransition temperature of all the jute composites,

Figure 8 Impact resistance of both the composites andthe hybrid composite. Figure 9 Storage modulus of the various composite and

hybrid composite samples obtained from the DMTA.[Color figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

Figure 10 Loss modulus for individual samples obtainedfrom the DMTA. [Color figure can be viewed in the onlineissue, which is available at wileyonlinelibrary.com.]



which might be due to the explanation given in theprevious paragraph.

The Tg values obtained in the loss modulus curves(Fig. 10) for all the composites were about 120�C,while they were between 90�C and 100�C for thehybrid composites. It can be concluded that the Tg

values obtained from the loss modulus curve arelower than those obtained from the tan d curve,which confirms the findings of many authors thatthe values of Tg from the tan d curve are alwaysexaggerated, whereas those from the loss moduluscurve are more reliable. Increase in storage and loss

modulus indicates better fiber–matrix adhesion. Theratio of E00 to E0 (loss modulus to storage modulus)gives the tangent of the phase angle d; tan d isknown as the damping and is a measure of energydissipation. Such parameters provide quantitative in-formation about the behavior of a material. The stor-age and loss modulus and the glass transition tem-perature can be increased by blending the matrixwith styrene, but in this study, neat resin was usedas a matrix.

Scanning electron microscopy

Microstructural analysis of the samples was donewith SEM. Figure 12(a) shows the microstructure ofthe tensile-fractured surface of the Lyocell compositeL. There was good fiber–matrix adhesion, as it wasvery difficult to see the fiber pull-out, but there wasfiber breakage instead. This indicated that the fiberwas well-embedded in the matrix. Fiber pull-outcould be seen in composites W1, W2, and W3 [seeFig. 12(b–d)], but the average fiber pull-out lengthwas relatively short, which also indicated that therewas good fiber–matrix adhesion but not as good asfor the Lyocell composite L.Figure 13(a–c) shows the hybrid composites that

had Lyocell and woven fabrics as reinforcements.The micrographs looked the same, and the effect ofhybridization with Lyocell fiber could be observed.

Figure 11 Tan d peak of the various composite andhybrid composite samples obtained from the DMTA.[Color figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

Figure 12 Scanning electron micrographs of composites L, W1, W2, and W3.



The fiber pull-out that was seen in composites W1,W2, and W3 could not be seen in composites W1L,W2L, and W3L. The tensile properties of the compo-sites and the hybrid composites agreed with theresults of the microstructural analysis. Lyocell fibercomposite L had a higher tensile strength, whichindicated good fiber–matrix adhesion.

CONCLUSIONS

There were some variations in the mechanical prop-erties, which were due to the different types of rein-forcements. In this study, the weave architectureswere the same for the woven fabrics but the differ-ence was the distance between adjacent roving weftsand warps, which contributed greatly to the surfaceweight of the fabric. Although one would haveexpected composite W2 to have better mechanicalproperties than the other woven jute compositesbecause the fiber was compact, due to the short dis-tance between the two adjacent roving wefts (about0.1 cm), this was not the case. The other woven jutefabrics had 0.2 and 0.5 cm between the adjacentwefts and warps, which led to lower surface weightand therefore a higher ply number in the composite.The mechanical properties of composites W1 andW3 were superior to that of W2.

The composites and the hybrid composites hadvery good mechanical properties. Hybridization withLyocell fiber increased the overall mechanical proper-ties of the composites but reduced their viscoelastic

properties. Although Lyocell offers better perform-ance at low cost, Lyocell fiber cannot replace wovenfabric but it can be used as hybrid to complement theproperties that are lacking in woven fabrics.Although the weight ratio of the jute–Lyocell fiber

was 60 : 40, the tensile and flexural properties mightbe improved if the ratio of the Lyocell fiber isincreased. Percentage elongation generally improvedwith inclusion of the Lyocell fiber. To increasetoughness in a composite, Lyocell fiber should beused as hybrid.

The authors thank the following people for their assistance inthe carding of the Lyocell fiber and also in the impact testing:Anders Bergner and Jan Johansson, Swerea IVFAB,Molndal,Sweden, and Haike Hilke, University of Boras, Sweden.Lenzing AG, Austria, is gratefully acknowledged for supply-ing the Lyocell fibers. We also thank Sung-Woo Cho, Univer-sity of Boras, Sweden, for his help in performing the SEManalysis.

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